Zero-IF transmitter employs homodyne or direct up-conversion method to transmit a pair of quadrature signals, i.e., signals that differ in phase by 90 degrees. The reference signal of the pair of quadrature signals, which is “in-phase,” is referred to as I signal. The signal that is shifted 90 degrees, and is in “quadrature” phase, is referred to as Q signal. During the direct up-conversion, the I and Q baseband signals are mixed with the in-phase and quadrature-phase components of a local oscillator signal to generate RF signals for transmission.
While generating the RF signals, it is important to maintain the amplitude relationship between the I and Q signals to ensure an accurate signal transmission, It is also important to maintain the phase relationship between the in-phase and quadrature-phase components of the local oscillator to prevent a phase skew. In reality, however, errors such as an IQ gain/phase imbalance existing in a zero-IF transmitter impairs the amplitude and phase relationship between the I and Q components of the RF signals. The transmission of the IQ RF signals is further complicated by a direct current (DC) offset present in IQ signals. This causes the local oscillator signal leaking into transmission (i.e., LO leakage).
A correction is attempted to compensate for impairments caused by IQ mismatch and DC offset by looping back the IQ RF signals through a feedback chain, and correlating the IQ baseband signal and the observed feedback signal to estimate relevant channel responses. Estimating relevant channel responses requires transmission of signals, from which feedback signals are obtained to perform a correlation to derive a channel response. Frequent transmission of signals to estimate channel responses may violate the regulatory specifications to adhere to FCC emission standards.